Multi-wavelength laser sources are key elements for a variety of applications such as wavelength division multiplexing (WDM), optical recording, color displays and optical color printing. Several methods have been proposed and patented for the realization of these multi-wavelength laser sources and the integration with modulators, as can be seen in U.S. Pat. No. 4,955,030 (Menigaux), U.S. Pat. No. 4,831,629 (Paoli), U.S. Pat. No. 4,993,036 (Ikeda), U.S. Pat. No. 5,384,797 (Welch), U.S. Pat. No. 5,519,721 (Takano). They can be realized by the non-uniform current pumping resulting in areas with different band filling, selective area epitaxy or quantum well intermixing. The last two methods can also be used to fabricate the lasers integrated with modulators.
However, many of the multiple wavelength laser sources or arrays fabricated rely on a large spatial separation between different wavelength elements, and therefore complex focusing optics are required to align the individual lasers to the optical fiber. It is desirable that all the laser emissions are combined monolithically together on a wafer so that the alignment restrictions can be eliminated. One method uses serially aligned semiconductor lasers each with a directional coupler [see U.S. Pat. No. 5,233,187, Japanese Patent 05090715, European Patent 496348, Sakata]. An inevitable drawback of this method is that there cannot be many wavelengths integrated together otherwise the device will be too long and the internal loss will be large. Also the grating must be optimized to act both as a directional coupler and the wavelength selective reflector. Another method is to combine parallel aligned laser arrays into one output port by use of appropriate waveguide alignment, such as the Y-junction coupler, rib waveguide coupler structures etc [See Japanese Patent 24209, 58175884, IEEE Photon. Technol. Lett. Vol. 7, pp. 944-946, 1995]. One problem arising from the combination of different wavelengths together is that the cross talks among these wavelengths are inevitable without additional treatment. The longer wavelength laser element will be optically pumped to lasing even only when the shorter wavelength laser element is electrically biased. The consequence is that each wavelength is not independently addressable. One method is to add an electroabsorption type modulator to each laser element. This method is a little complicated since it needs a lot of additional electrodes to control each modulator.
In this patent we propose a new method, the introduction of an isolator, to provide the needed isolation among different wavelength laser elements. It does not need additional electrodes so it is simple to fabricate and easy to operate. We also provide the method of fabricating the said isolator and other integrated optoelectronic devices.